The invention relates to a method for manufacturing an optical device with a defined total device stress. More particularly, the invention relates to a method for manufacturing an optical waveguide wherein the waveguide core stress and the cladding layer stress sum up to a total device stress with a desired distribution, more particularly, this distribution being such that the optical mode(s) in the waveguides do not experience any birefringence and the polarization dependence is minimized or such that the birefringence and the polarization dependence are set to a desired, defined value.
In the article xe2x80x9cCharacterization of Silicon-Oxynitride Films deposited by Plasma Enhanced CVDxe2x80x9d by Claassen, v.d. Pol, Goemans and Kuiper in J. Electrochem. Soc.: Solid state science and technology, July 1986, pp 1458-1464 the composition and mechanical properties of silicon-oxynitride layers made by plasma-enhanced deposition using different gas mixtures are investigated. It is stated that the mechanical stress strongly depends on the amount of oxygen and hydrogen incorporated in the layer. Heat treatment at temperatures higher than the deposition temperature leads to a densification of the film due to hydrogen desorption and cross-linking.
In xe2x80x9cTemperature dependence of stresses in chemical vapor deposited vitreous filmsxe2x80x9d by Shintani, Sugaki and Nakashima in J. Appl. Phys. 51(8), August 1980, pp 4197-4205 it is shown that in vitreous silicate glass depending on deposition background pressure different components of tensile and compressive stress occur. Also a hysteresis of the stress is observed.
In xe2x80x9cStress in chemical-vapor-deposited SiO2 and plasma-SiNx films on GaAs and Sixe2x80x9d by Blaauw in J. Appl. Phys. 54(9), September 1983, pp 5064-5068 stress in films of CVD-SiO2 and plasma-SiNx on GaAs is measured as a function of temperature. Different properties of the stress are observed depending on, e.g., film thickness, doping and annealing parameters.
xe2x80x9cStress in silicon dioxide films deposited using chemical vapor deposition techniques and the effect of annealing on these stressesxe2x80x9d by Bhushan, Muraka and Gerlach in J. Vac Sci. Technol. B 8(5), September/October 1990, pp 1068-1074 deals with in situ measured stress as a function of temperature. Different deposition techniques are investigated and in PECVD silica films on silicon substrates a change of the stress sign from compressive to tensile is observed with rising annealing temperature.
U.S. Pat. No. 4,781,424 is related to a single mode optical waveguide having a substrate, a cladding layer formed on the substrate, a core portion embedded in the cladding layer, and an elongated member for applying a stress to the core portion or a stress relief groove for relieving a stress from the core portion in the cladding layer along the core portion. The position, shape and material of the elongated member or the groove are determined in such a way that stress-induced birefringence produced in the core portion in accordance with a difference in thermal expansion coefficient between the substrate and the single mode optical waveguide is a desired value.
In U.S. Pat. No. 5,502,781, integrated optical devices which utilize a magnetostrictively, electrostrictively or photostrictively induced stress to alter the optical properties of one or more waveguides in the device are disclosed. The integrated optical devices consist of at least one pair of optical waveguides preferably fabricated in a cladding material formed on a substrate. A stress applying material, which may be a magnetostrictive, electrostrictive or photostrictive material, is affixed to the upper surface of the cladding material near at least one of the optical waveguides. When the appropriate magnetic, electric or photonic field is applied to the stress applying material, a dimensional change tends to be induced in the stress applying material. The constrained state of the stress applying material, however, caused by its adhesion to the cladding material, causes regions of tensile and compressive stress, as well as any associated strains, to be created in the integrated optical device. By positioning one or more optical waveguides in a region of the device which will be subjected to a tensile or compressive stress, the optical properties of the stressed waveguide may be varied to achieve switching and modulation. Latchable integrated optical devices are achieved by utilizing a controlled induced stress to xe2x80x9ctunexe2x80x9d one or more waveguides in an integrated optical device to a desired refractive index or birefringence, which will be retained after the field is removed.
U.S. Pat. No. 4,358,181 discloses a method of making a preform for a high numerical aperture gradient index optical waveguide. Therein the concentration of two dopant constituents is changed during fabrication. Concentration of the first dopant, germanium, is changed radially as the preform is built up in order to produce the desired radial refractive index gradient. The concentration of the second dopant, boron, is changed radially to compensate for the radial change in thermal expansion coefficient caused by the varying Ge concentration. B is added to the cladding layer to make the thermal expansion coefficient of the cladding equal to or greater than the composite thermal expansion coefficient of the core. The magnitude of residual tension at the inner surface caused by thermal expansion gradients is reduced and premature cracking of the preform is eliminated.
Disclosed in U.S. Pat. No. 4,724,316 is an improved fiber optic sensor of the type in which a fiber optic waveguide component of the sensor is configured to be responsive to an external parameter such that curvature of the fiber optic waveguide is altered in response td forces induced by changes in the external parameter being sensed. The alteration of the curvature of the fiber optic waveguide causes variations in the intensity of light passing therethrough, these variations being indicative of the state of the external parameter. The improvement comprises coating material covering the exterior portion of the fiber optic waveguide, the coating material having an expansion coefficient and thickness such that distortion of the fiber optic waveguide caused by thermally induced stresses between the coating material and the glass fiber is substantially eliminated. Also disclosed is a support member for supporting the curved fiber optic waveguide, the support member and fiber optic waveguide being configured and arranged to minimize the effects of thermal stress tending to separate the waveguide from the support member.
According to a first aspect of the invention herein is provided a method for manufacturing an optical device with a defined total device stress and a therefrom resulting defined birefringence, which device is easy to manufacture and at the same time provides a high precision in the resulting birefringence value with the final intent to obtain a defined optical polarization dependence for the optical mode propagating in the device.
The total device stress is substantially determined by the waveguide core stress and the waveguide cladding stress, also referred to as cladding layer stress, both of which can be influenced by thermal annealing processes the device may be subjected to. The waveguide core stress and the waveguide cladding stress can be tuned independently, for example, by variation of the temperature of the respective annealing step. The tuned state, namely, the state when the desired waveguide core stress, the desired waveguide cladding stress, and the desired device birefringence have been achieved, will remain after the annealing step(s). The tuned state can be such that the optical mode(s) in the waveguides do not experience any birefringence and the polarization dependence is minimized, or such that the birefringence and the polarization dependence is set to a desired non-zero-value, e.g., to build an optical mode converter.
The waveguide core stress is typically predetermined by parameters such as the desired refractive index and the maximum allowed optical losses. The cladding layer stress counteracts the waveguide core stress whereby the overall device stress and the device birefringence can be controlled.
The vice versa effect can also be utilized. Possible restrictions that determine the stress of the cladding layer can be taken into account and the desired value of the device birefringence can be approached or set via a tunability of the waveguide core stress.
The profile of the upper-cladding-layer annealing step as specified in claim 1 has the advantage that it can be used to tune the upper-cladding-layer stress such that the optical mode(s) in the waveguide core do(es) not experience any birefringence and the polarization dependence is minimized, or such that the birefringence and the polarization dependence are set to a desired non-zero-value.
The profile of the waveguide core-annealing step as specified in claim 9 has the advantage that desired refractive index can be achieved and furthermore a low optical loss at 1550 nm is realized while the resulting waveguide core stress is combined with a resulting cladding layer stress to achieve a desired birefringence.
The profile of the upper-cladding-layer annealing step specified in claim 1 differs from the profile of the core-annealing step specified in claim 9, in particular in the heating and cooling rates. This difference is due to the fact that in the upper-cladding-layer annealing step the results of the core-annealing step shall only be influenced so far that the overall waveguide device stress and with it the birefringence are determined. The parameters are hence selected not to introduce effects that negatively influence the device functionality and in particular not to risk device cracking or cause undesired additional stress. At the same time the parameters achieve a relatively short overall process time.
In contrast to other approaches to avoid stress in planar devices like fabricating stress-relief grooves, no complex processing steps like an additional lithographic mask or etching step are required here. Annealing steps as described here are easily controllable. They only introduce negligible additional complexity and costs.
A difference in the refractive index for TE and TM modes in a waveguiding structurexe2x80x94a birefringencexe2x80x94leads to polarization-dependent effects in an optical component. Most of today""s optical communication systems using single mode waveguides are desired to have negligible polarization dependence because in that case polarization control can be neglected. Therefore, the birefringence is one of the most important factors that determines the performance of a waveguide-type optical component part.
The major contribution to the birefringence is induced by the stress in the layered stack of the waveguiding material, i.e., birefringence induced by the waveguide geometry is typically only a few times 10xe2x88x924 whereas the stress-induced birefringence is in many cases an order of magnitude larger. A significant process step in fabricating the waveguide is the thermal annealing step of the core layer, comprising preferably SiliconOxyNitride, i.e. SiOxNy, that is mainly performed to reduce the hydrogen-bond induced optical loss. This annealing step is accompanied by the introduction of stress in the waveguide substantially due to the difference of the thermal expansion coefficient of the layers and the substrate. To control the stress-induced birefringence with a high degree of accuracy it is hence of importance to accurately control for a given material choice of the waveguide core, waveguide cladding, and substrate, the thermal annealing step(s) that the device is subjected to.
Therefore according to a first aspect of the invention herein is provided a method for manufacturing an optical device that by means of one or more annealing steps has a tunable total device stress and a therefrom resulting tunable birefringence and a therefrom resulting controlled optical polarization dependence.
The method comprises a first step of providing a lower cladding layer of preferably amorphous SiO2, that may be doped with elements like Boron and/or Phosphorous, with a first refractive index and a second step of providing above the lower cladding layer an upper cladding layer of preferably amorphous SiO2, that may be doped with elements like Boron and/or Phosphorous, with a second refractive index, and being manufactured from a material that is tunable in its stress. In a third step between the lower cladding layer and the upper cladding layer an optical waveguide core is manufactured comprising preferably SiliconOxyNitride, i.e. SiOxNy, and having a third refractive index that is larger than the first refractive index and than the second refractive index and having a waveguide core stress.
In a fourth step the waveguide upper cladding is thermally annealed by first keeping the upper cladding layer at a first temperature between 400 and 600xc2x0 C. for a preparation time of at least 0.5 hours, then raising the temperature to a second temperature between 600 and 1280xc2x0 C. with a heating rate between 5 and 20 K/min, maintaining the second temperature for an annealing time between 2 and 5 hours, and lowering the temperature to a third temperature between 300 and 600xc2x0 C. with a cooling rate between xe2x88x920.5 and xe2x88x9210 K/min, after which the temperature is lowered to a fourth temperature above 10xc2x0 C.
Thereafter the upper cladding layer has a well defined cladding layer stress that together with the waveguide core stress results in the total device stress.
In a fifth step the waveguide core can be thermally annealed by first keeping the waveguide core at a first temperature between 400 and 600xc2x0 C. for a preparation time of at least 0.5 hours, then raising the temperature to a second temperature between 1100 and 1280xc2x0 C. with a heating rate between 2 and 20 K/min, maintaining the second temperature for an annealing time between 2 and 4 hours, and lowering the temperature to a third temperature between 300 and 600xc2x0 C., with a cooling rate between xe2x88x920.5 and xe2x88x923 K/min, after which the temperature is lowered to a fourth temperature above 10xc2x0 C.
The typical step sequence for manufacturing the optical device will be: first, third, fifth, second and then fourth step. This is a sequence which runs from bottom to top, i.e. providing e.g. on a substrate, the lower cladding layer, then depositing thereupon the optical waveguide core, optionally annealing it, depositing the upper cladding layer and annealing it.
The tuned state, namely, the desired waveguide core stress, the cladding layer stress, and the desired device birefringence, will remain after the annealing step(s).
With this method of manufacturing an optical device the stress of the upper cladding layer can be advantageously used to significantly reduce or even compensate the stress of the waveguide core such that the total device stress is minimized. This leads to a minimized birefringence that is fully compensated by properly designing the waveguide core geometry. The extinction of the birefringence results in a minimized polarisation dependence of the optical device. With this method of manufacturing an optical device the birefringence can also be tuned to a defined non-zero value resulting in a defined polarisation dependence of the optical device.
A more complete understanding of the present invention as well as further features and advantages of the invention will be obtained by reference to the detailed description and drawings.